1. Field of the Invention
The present invention relates to a switching converter. More particularly, the present invention relates to a switching converter for making a non-control detection discrimination signal invalid in an overload state during a soft start operation, thereby preventing a malfunction to carry out a correct protecting operation.
2. Description of the Related Art
An example of a switching converter according to a conventional embodiment will be simply described with reference to the drawings. FIG. 1 shows the structure of a switching converter, for example, a current resonance bridge converter.
In FIG. 1, MOS field effect transistors 12 and 13 are mutually switching-driven and a voltage is induced to a secondary will Ns of a transformer 15 through the current resonance of a series resonance circuit which comprises inductances L15a and L15b of the transfer 15 and a capacitor 16. A DC output voltage Vs obtained by rectifying and smoothing the voltage is supplied to a load 20 and an error amplifier 21.
A photocoupler 23 is driven by the error amplifier 21 in response to a difference between the Dc output voltage Vs and a reference voltage Vref. Consequently, the frequency of an oscillation signal Sosc generated by the oscillating circuit 30 is varied. In a driving circuit 33, moreover, driving signals SD1 and SD2 are generated based on the oscillating signal Sosc and are supplied to the transistors 12 and 13 so that the DC output voltage Vs is controlled to be equal to the reference voltage Vref.
When the operation of the switching converter is started, a capacitor 36 connected to a soft start control circuit 35 is charged and a soft start control signal SFC corresponding to the voltage level of a terminal voltage Vst of the capacitor 36 is generated by the soft start control circuit 35 and is supplied to the oscillating circuit 30.
Moreover, if the positive polarity terminal of a comparator 28 is referred to as a connecting point Q, an overload state is brought and the voltage level of the DC output voltage Vs is reduced. Consequently, a discrimination voltage Va on the point Q becomes lower than a non-control discrimination reference voltage Vdr. For this reason, it is possible to judge whether it is the overload state or not, based on a non-control detection signal SLA sent from the comparator 28 in a latch control circuit 40 as an oscillation driving control means. If it is judged that it is the overload state, a driving control signal SDC is supplied to the driving circuit 33 to stop the generation of the driving signals SD1 and SD2 in the driving circuit 33. Consequently, the operation of the switching converter can be stopped during the overload.
In a conventional current resonance type converter, the oscillation frequency of the oscillating circuit 30 is increased through the soft start control signal SFC sent from the soft start control circuit 35 during starting. Consequently, an operation is started with a high primary resonance impedance and the transistors 12 and 13 operate in a safe operation region.
FIG. 2 shows the operation of each portion during the starting. FIG. 2A shows the driving signal SD1, FIG. 2B shows the driving signal SD2, FIG. 2C shows the terminal voltage Vst of the capacitor 36 for determining a soft start period, and FIG. 1D shows the discrimination voltage Va of the connecting point Q which is varied according to the DC output voltage Vs.
When the switching converter is started at a time t81, the supply of the driving signals SD1 and SD2 are started as shown in FIGS. 2A and 2B. Moreover, the charging operation of the capacitor 36 is started so that the terminal voltage Vst of the capacitor is raised as shown in FIG. 2C. Furthermore, the discrimination voltage Va on the point Q shown in FIG. 8D is also raised.
Then, the DC output voltage Vs is lower than the output reference voltage Vref at a time t82 immediately after the soft start is started. Therefore, a phototransistor 23b is in a cut-off state. However, a voltage Vcc is applied to the point Q through a constant current source 27. Therefore, the discrimination voltage Va reaches a voltage level Ldr of the reference voltage Vdr. For this reason, this is erroneously recognized as the overload state in a latch control circuit 40. There is a fear that the transistors 12 and 13 which should properly continue the operation might be stopped.
In order to solve the above-mentioned problem, a capacitor 28y for preventing malfunction is connected to the connecting point Q and the capacitor 28y is charged when the switching converter is started. Thus, the discrimination voltage Va is controlled so as not to exceed the voltage level Ldr for a soft start period. More specifically, the capacity of the capacitor 28y should be determined such that the charging time of the capacitor 28y becomes longer than that of the capacitor 36.
However, the capacities of the capacitor 28y and the capacitor 36 for determining the soft start period are different great individually. Therefore, it is a matter of course that there is a great difference individually in the charging period of the capacitor 28y and the soft start period which is the charging period of the capacitor 36. For this reason, the charging time of the capacitor 28y should be maintained to be sufficiently long in order to surely prevent the malfunction of the latch control circuit 40. Consequently, the charging period of the capacitor 28y remains after the soft start period and the overload state which should be properly detected cannot be detected after the soft start period. Therefore, a timing for supplying the non-control detection signal SLA is delayed. Accordingly, it is not easy to determine the capacity of the capacitor 28y while maintaining a balance with the charging time of the capacitor 36.
The invention solves such a conventional problem and particularly proposes that the malfunction of the switching converter is prevented and a correct protecting operation is carried out.
In order to solve the above-mentioned problem, a first aspect of the present invention is directed to a switching converter for switching a transistor in response to a driving signal sent from oscillation driving means to obtain a desirable DC output voltage, comprising:
overload detecting means for detecting an overload state and outputting a non-control detection discrimination signal;
soft start control means for controlling the frequency of the oscillation signal of the oscillation driving means in a soft start period which is a predetermined period and starts during starting-up, thereby carrying out a soft start operation for raising gradually the DC output voltage to a desirable voltage level; and
oscillation driving control means for controlling the oscillation driving means based on the non-control detection discrimination signal to stop the driving signal, thereby carrying out a protecting operation and making the non-control detection discrimination signal invalid during the soft start operation.
Moreover, a second aspect of the present invention is directed to the switching converter characterized in that the oscillation driving control means stops the driving signal for a predetermined period after the overload state is detected, based on the non-control detection discrimination signal, and then controls the soft start control means, thereby carrying out the soft start operation to executing an intermittent operation.
Furthermore, a third aspect of the present invention is directed to the switching converter characterized in that the soft start control means sets the soft start period by charging a capacitor, and
the oscillation driving control means discharges of the capacitor when the soft start operation in the intermittent operation is started.